Metal base transistors have been widely investigated because they can provide better high frequency performance than conventional bipolar transistors. A metal base transistor uses "hot electron" transport from emitter to collector. As is well known to those having skill in the art, a hot electron is an electron with energy more than a few kT above the Fermi energy, where k is Boltzmann's constant and T is the lattice temperature. Hot electron devices may be modeled after the vacuum tube diode because the transported electrons are not in thermal equilibrium. Since these hot carriers are transported through the base at high velocities, a short transit time and a potentially large current gain may be obtained.
A metal base transistor has been fabricated by embedding a 90 .ANG.ngstrom thick gold film between epitaxial layers of silicon and germanium. See the textbook entitled Physics of Semiconductor Devices by Sze, Section 3.64, p. 184, published by John Wiley, 1981. As is well known to those having skill in the art, an epitaxial layer is a monocrystalline layer which is lattice matched to an adjacent monocrystalline layer to create a monolithic monocrystalline two-layer structure. In fabricating the silicon/germanium metal base transistor, a 90 .ANG.ngstrom gold film layer was grown on a single crystal silicon film using molecular beam epitaxy, and a monocrystalline germanium film was grown on the monocrystalline gold film.
The permeable base transistor may be considered a variant of the metal base transistor. In a permeable base transistor, an ultrafine metal grid is formed between epitaxial semiconductor films. The metal forms a Schottky (rectifying) barrier with the surrounding semiconductor material, and the built-in voltage totally depletes the spaces in the grid. When a sufficient positive bias is applied to the base electrode, the depletion layer shrinks and a conductive path forms between the collector and emitter. The cutoff frequency of such a device is a strong function of the grid size and is presently close to 40 GHz. Cutoff frequencies close to 200 GHz are possible if the grid size is decreased to about 500.ANG.. See Chapter 11 of the textbook by M. Shur entitled GaAs Devices and Circuits, pp. 611-615, published by Plenum Press, 1987.
A permeable base transistor has been fabricated using an n+ gallium arsenide substrate, on which an n type gallium arsenide emitter layer is formed. A patterned metal film, such as tungsten with a thickness of 200.ANG. and a Schottky barrier height of 0.8V, is formed on the n-type gallium arsenide emitter layer. The tungsten film has a conductive line width of 1600.ANG. and spaces between the lines of about 1600.ANG.. An n type collector layer is then formed on the tungsten conductive lines and on the underlying gallium arsenide emitter layer between the lines. X-ray lithography and epitaxial overgrowth were used to epitaxially form the metal permeable base on the underlying n type gallium arsenide layer and form the overlying gallium arsenide layer on the metal base. See the above cited textbook to Sze, Section 3.6.5, pp. 84-186. With such a fine base grating and an appropriate carrier concentration, barrier limited current flow can exist at high current densities, resulting in a large transconductance and a large maximum operating frequency.
As described above, metal base and permeable base transistors have been fabricated in silicon, germanium and gallium arsenide. However, diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than silicon, germanium or gallium arsenide. Diamond provides a higher energy bandgap, a higher breakdown voltage and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using silicon, germanium or gallium arsenide. Silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, i.e., diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. In particular, the art has investigated diamond metal base/permeable base transistors.
FIG. 4A of a publication by M. W. Geis et al. entitled Summary Abstract: Device Applications of Diamond, Journal of Vacuum Science Technology, Vol. A6, No. 3, May-June, 1988, pp. 1953-1954, describes a diamond transistor in which a grating pattern is formed in a boron doped diamond layer, and the resultant trenches are lined with chemically vapor deposited silicon dioxide. An aluminum layer is formed at the bottom of each trench, with an ohmic collector contact formed to the top of the trenches and an ohmic emitter contact formed at the bottom of the boron doped diamond layer. As described in this publication, the device has a transconductance of 30.mu.S mm.sup.-1, which results from parasitic resistance of the device associated with the high substrate resistance. See also FIG. 2(a) of a publication by M. W. Geis entitled Diamond Transistor Performance and Fabrication, Proceedings of the IEEE, Vol. 79, No. 5, May 1991, pp. 669-676, which identifies this transistor as a vertical field effect transistor. Notwithstanding the above publications, there exists a need for improved metal base and permeable base bipolar transistor structures which are particularly adapted for fabrication in diamond.